Pixel based machine for patterned wafers

ABSTRACT

A method is provided for the detection of defects on a semiconductor wafer by checking individual pixels on the wafer, collecting the signature of each pixel, defined by the way in which it responds to the light of a scanning beam, and determining whether the signature is that of a faultless pixel or of a pixel that is defective or suspect to be defective. An apparatus is also provided for the determination of such defects, which comprises a stage for supporting a wafer, a laser source generating a beam that is directed onto the wafer, collecting optics and photoelectric sensors for collecting the laser light scattered by the wafer in a number of directions and generating corresponding analog signals, an A/D converter deriving from said signals digital components defining pixel signatures, and selection systems for identifying the signatures of suspect pixels and verifying whether the suspect pixels are indeed defective.

FIELD OF THE INVENTION

[0001] The invention relates to the inspection of surfaces, particularlythe surfaces of semiconductor wafers, intended for the detection ofpossible defects, particularly due to the presence of particles. Moreparticularly the invention relates to the control of semiconductormanufacturing processes, particularly Quality Control, ProcessMonitoring and Control and Catastrophe Detection. The invention furthercomprises method and apparatus for the inline control of waferproduction and the immediate recognition of any fault or irregularitiesin the production line.

BACKGROUND OF THE INVENTION

[0002] The detection of defects and/or of the presence of foreignsubstances on semiconductor wafers has received considerable attentionin the art. Defects can be caused by an imperfect production of thedesired pattern. Further, particles of various kinds may adhere to awafer surface for a number of reasons.

[0003] The inspection process can be carried out on bare wafers, viz.wafers that have not yet been patterned, or on patterned wafers. Thisinvention relates primarily to the inspection of patterned wafers.

[0004] Prior art devices are used in order to detect defects andparticles of the type described above in patterned wafers. Examples ofprior art apparatus comprise devices based on the direct comparison ofdifferent dies. Such apparatus, which will be further referred to belowwith respect to specific references, presents the followingdrawbacks: 1) it is relatively very expensive, as it requires highmechanical precision; 2) it has low throughput; 3) it has a largefootprint; 4) it needs an expert operator; 5) it is not suitable forinline inspection (i.e., it operates on wafers which have beenpreviously removed from the fabrication line), and therefore isunsuitable for purposes of process control and monitoring, of the kindaddressed by the present invention; 6) prior art devices arenon-isotropic devices, i.e. they require a very precise alignment of thearticle being inspected. These facts impose constructive and operativeconstraints on the apparatus and on the inspection method.

[0005] U.S. Pat. No. 4,731,855, to Kyo Suda et al, includes in itsBackground of the Invention a list of various methods for performingsemiconductor wafer inspections, and said list is incorporated herein byreference. One of said methods involves scanning the wafer surface witha laser beam and analyzing the number and direction of diffractionlights, produced by the pattern edges, by means of a plurality of lightdetection cells arranged cylindrically.

[0006] U.S. Pat. No. 4,345,312, to Toshikazu Yasuye et al, discloses apattern inspecting method which comprises picking up an image from anarticle having a preset pattern whereby to extract the data of thepattern to be inspected, converting said data into a bit matrix ofbinary values, and comparing said matrix with a reference matrixrepresenting an ideal pattern, to disclose any discrepancy between thepattern of the article and the ideal one.

[0007] U.S. Pat. No. 4,342,515, to Masakuni Akiba et al, discloses aninspection apparatus for determining the presence of foreign matters onthe surface of a wafer, which apparatus includes a beam generatorportion which projects a collimated beam towards the object to inspectit from a side thereof, and a mechanism which senses light reflectedfrom the surface of the object, through a polarizer plate. Such methods,however, are obsolete inasmuch as they cannot be used with today'swafers having a design rule of 0.5 μm or less.

[0008] The same principle is used in several prior art methods andapparatus. Thus, in U.S. Pat. No. 4,423,331, to Mitsuyoshi Koizumi etal, the light reflected from the wafer surface is directed to aphotoelectric tube and defects are detected by the irregularities of thevoltage current outputted by the tube.

[0009] U.S. Pat. No. 4,669,875, to Masataka Shiba et al, makes referenceto the aforesaid U.S. Pat. No. 4,342,515, and proposes a method andapparatus based on the same principle, in which a polarized laser beamirradiates the substrate from directions inclined with respect to theperpendicular to its surface and linearly scans said surface; and lightreflected from foreign particles is detected by a polarized lightanalyzer sand a photoelectric conversion device.

[0010] The aforesaid U.S. Pat. No. 4,731,855 discloses a method ofdetecting defects, e.g. foreign particles, in which the diffractionlight reflected from a wafer surface is analyzed by distinguishingbetween normal and abnormal directions. An ideal pattern formed on awafer reflects diffraction lights in determined directions, at certainangles, which are considered normal directions. On the other hand,foreign particles reflect the light in other, abnormal directions.Reflection of light in abnormal directions indicates a departure of thepattern formed on the wafer from the real pattern, and thereforepossible defects. In the invention of this reference, the abnormaldirection signals are so applied as to determine whether they representa true defect or a practically acceptable defect. Again, this method isobsolete due to the design rule of less than 1 μm.

[0011] U.S. Pat. No. 4,814,596, to Mitsuyoshi Koizumi et al, applies thesaid principle of analyzing polarized reflected light to identifydefects. It cites the aforesaid U.S. Pat. No. 4,342,515 as well asJapanese Patent Applications Publication Nos. 54-101390, 55-94145 and56-30630. In the apparatus of this reference, an S-polarized beam isarranged to illuminate the pattern present on the wafer. Since theirregularities in the surface of the pattern are sufficiently small, theS-polarized light is preserved in the reflected light. An analyzer isused to cut the S-polarized light in the path of the reflected light, sothat, if the reflected light includes a P-polarized light, this latteris detected by a photoelectric conversion element, indicating thepresence of particles on the wafer.

[0012] U.S. Pat. No. 4,628,531, to Keiichi Okamoto et al, discloses apattern checking apparatus, which reveals by a primary selection thepresence of defects that may be tolerable or not, defined as “candidatedefects”. The wafers having such defects are passed to a secondaryselection, which distinguishes between those that are not defects in apractical sense and are acceptable, and those that are not acceptable.False alarms, viz. the detection in the primary selection of apparentdefects, which are revealed in the secondary selection not to be realdefects, are said to be caused, in prior art methods based on thecomparison of patterns, by an imperfect registration of the patterns tobe compared.

[0013] Another method of the prior art relates to inspection apparatusemploying a planar array of individually addressable light valves foruse as a spatial filter in an imaged Fourier plane of a diffractionpattern, with valves having a stripe geometry corresponding to positionsof members of the diffraction pattern, blocking light from thosemembers. The remaining valve stripes, i.e. those not blocking light fromdiffraction order members, are open for transmission of light. Lightdirected onto the surface, such as a semiconductor wafer, formselongated curved diffraction orders from repetitive patterns of circuitfeatures. The curved diffraction orders are transformed to linear ordersby a Fourier transform lens. Various patterns of stripes can be recordedand compared. Related discussion can be found in U.S. Pat. Nos.4,000,949 and 4,516,833.

[0014] U.S. Pat. No. 5,699,447 discloses and claims an apparatus whichcomprises first examining means for examining in a first phase thecomplete surface of the wafer with an optical beam of small diameter andfor outputting information, indicating inspected locations on thearticle's surface having a high probability of a defect, storage meansfor storing the output of the first examining means, and secondexamining means for examining in a second phase and with a relativelyhigh spatial resolution only the locations having a high probability ofa defect and for outputting information indicating the presence orabsence of a defect in said locations. The first examination phase iseffected by making a comparison between the pattern of the inspectedwafer and another pattern, serving as a reference pattern; and thesecond examination phase is carried out by a similar comparison toidentify the locations in which the comparison shows such differences asto indicate the presence of a defect.

[0015] The methods and apparatus of the prior art have severaldrawbacks, partly discussed in the cited references, such as errors dueto faulty registration and other causes, false alarms consisting in thedetection of defects that are only apparent, and so on. All of them,further, have the common drawback of requiring complex apparatus, withhigh mechanical precision, and requiring long operation times and havingtherefore a low throughput.

[0016] It is therefore a purpose of this invention to eliminate thedrawbacks of the prior art method and apparatus for the inspection ofpatterned semiconductor wafers, and particularly for determining thepresence of particles of foreign substances.

[0017] It is another purpose of this invention to provide such a methodand apparatus that operate at a much higher speed than prior artapparatus and with a much higher throughput.

[0018] It is a further purpose of this invention to detect the defectsor suspected defects of surfaces, particularly of patterned,semiconductor wafers, by a system that does not require comparison ofpatterns.

[0019] It is a still further purpose of this invention to detect saiddefects or suspected defects by an inspection or testing of the pixelsof the surface.

[0020] It is a still further purpose of this invention to detect saiddefects or suspected defects by an analysis of the optical response ofthe pixels of the surface to a scanning beam.

[0021] It is a still further purpose of this invention to provide awafer control method that is not based on a comparison of patterns, butis a pixel-based inspection.

[0022] It is a still further purpose of this invention to provide awafer control method and apparatus that are completely automatic andeliminate almost all possibility of human error.

[0023] It is a still further purpose of this invention to provide such amethod and apparatus which are highly flexible or, in other words, thatcan be operated in such a way as to achieve the precision that isrequired in any particular processing situation.

[0024] It is a still further purpose of this invention to provide amethod and apparatus for controlling the semiconductor wafers inline andimmediately recognizing any failures or irregularities in the productionprocess and apparatus.

[0025] It is a still further purpose of this invention to provide amethod and apparatus that permit to localize on the wafer surface theposition of any suspected defects.

[0026] It is a still further purpose of this invention to provide amethod and apparatus for a primary control of semiconductor wafers thatfacilitates carrying out a subsequent operation called herein a “vectordie-to-die comparison”.

[0027] By the expression “vector die-to-die comparison” (abbreviated asVDDC) is meant, in this specification and claims, an operation thepurpose of which to determine which of the suspected defects representvalid pattern data and which represent real defects. The preferredembodiment described herein requires firstly transforming the polarcoordinates of the wafer inspecting apparatus—hereinafter “the machinecoordinate system”—to the Cartesian coordinates of a system hereinafterdefined—“the die coordinate system of the wafer”—Then, deriving from thecoordinates that define the suspect pixels' location in the machinecoordinate system the coordinates that define said location in the diecoordinate system. Finally, the VDDC is an operation for discriminating,between suspect data that are actually produced by the wafer pattern andsuspect data that are produced by real contamination by particles—all aswill be fully explained hereinafter.

[0028] It is a still further purpose of this invention to provide amethod and apparatus for the analysis of surfaces, even if they are notsurfaces of patterned semiconductor wafers.

[0029] It is a still further purpose of this invention to provide anoptical head which comprises, in a structural unit, all the opticalelements required for irradiating the pixels of the surface with thebeam used for scanning and collecting their optical response in theparticular manner of this invention, as hereinafter described.

[0030] It is a still further purpose of this invention to provide anapparatus which effects the control of the pixels by a combination ofsuch an optical head and means for displacing the surface relative toit.

[0031] Other purposes and advantages of the invention will appear as thedescription proceeds.

SUMMARY OF THE INVENTION

[0032] The invention, both as to method and apparatus, is based on theprinciple of inspecting all or part of the individual pixels of thepatterned wafers under control, without comparing patterns or needingspecific information about the patterns. In other words, the inventionis based on the principle of detecting suspected pixels, viz. pixelsthat show signs of having a defect, particularly the presence of foreignparticles, without reference to the pattern to which the pixel belongsor to the position of the pixel on the wafer and without comparisonbetween patterns. This inventive inspection method is termed herein“design rule check”. Although reference will be made herein to patternedsemiconductor wafers, the analysis of which is the primary purpose ofthe invention, it will be apparent that the invention can be applied ingeneral to the analysis of different surfaces, particularly of anysurfaces not patterned or having patterns the dimensions of which aresimilar to those of wafer patterns, e.g. in the order of microns orfractions of microns.

[0033] The method and apparatus of the invention can be used “inline”,viz. is suitable to be integrated with the production process tool,using the same wafer handling and interface system, and can operate asan integrated particle monitor to provide a constant check of the wafersproduced, and in this way will detect any irregularities or defects thatmay arise in the production line. Sometimes unforeseen phenomena mayoccur in the production line that are so far-reaching as to render itsfurther operation impossible or useless. It is important to detect suchphenomena, which may be termed “catastrophic”, as soon as possible, andthis invention permits to do so. These inline checks are renderedpossible for the first time in the art by the high speed of thepixel-based inspection method and the moderate cost and footprint of theapparatus.

[0034] According to an aspect of the invention, the same comprises amethod for the determination of defects, particularly the presence offoreign particles, in patterned, semiconductor wafers, which comprisessuccessively scanning the individual pixels, defining the signature ofeach pixel, and determining whether said signature has thecharacteristics of a signature of a faultless or of a defective, orsuspected to be defective, pixel.

[0035] In some embodiments of the invention, the determination of thecharacteristics of the pixel signatures is preceded by preliminary stepsof evaluation of the characteristics of the individual signals that makeup the signature, which permit to conclude that certain signaturescannot belong to defective pixels, and therefore require no furtherprocessing, whereby to reduce the amount of data that must be processed.Therefore the method of the invention may comprise defining thesignature of each pixel, evaluating each signal of each signature, and,based on said evaluation, excluding a number of signatures from furtherprocessing. Preferably, the pixels are optically scanned by means of anilluminating beam and their signature is defined by their opticalreaction to the illuminating light. In this case, the variousembodiments of the method of the invention are characterized by thefollowing features:

[0036] I—The type of light being used;

[0037] II—The physical and geometric parameters of the illumination;

[0038] III—The property and/or parameters by which the optical reactionof the pixels, and therefore their signature, is characterized;

[0039] IV—The physical and geometric parameters of the detection of saidoptical reaction.

[0040] I—The Type of Light Being Used

[0041] According to the invention, one can use laser beams or lightproduced by other sources, such as flash lamps, fluorescence lamps,mercury lamps, etc. Laser beams can be produced e.g. by diode lasers andhave any wavelength, e.g. 400 to 1300 nm. The choice of the appropriatewavelength can be carried out by skilled persons in any case, so as toproduce optimization for a given material or pattern. Relatively longwavelength (e.g. 600-810 nm) are generally preferred because of the highenergy fluence achievable. Short wavelengths can be preferred fordetecting small particles and for finer design rules. Laser beams canalso be produced by non-diode generators, of any wavelength from IR todeep UV. The illuminating radiation may be narrow band or wide band(important for spectral analysis). It can be coherent or non-coherent,polarized or non-polarized. As to fluence, it can be CW, pulsed orquasi-CW. One or a plurality of light beams can be used.

[0042] II—The Physical and Geometric Parameters of the I llumination

[0043] 1. The number of the illumination sources can be changed.

[0044] 2. The geometric placement of the illumination sources can bechanged.

[0045] 3. The size and form of the light source and of the illuminatedspot can be changed.

[0046] 4. The way in which the illumination light is delivered can bechanged.

[0047] Important changes can arise from changing the size of theilluminated spot with respect to a given pattern. A spot of 5 squaremicrons will provide a completely different set of signatures than aspot of 75 square microns, and different discrimination capability. Someuseful light source forms are a point source, a ring source, a largeaperture source, and a line source. It may sometimes be beneficial toilluminate through the wafer (or through another article, when such isbeing inspected, such as a reticle or some other transparent article)with a relatively large wavelength (more than 1 micron). Thus, one couldilluminate from beneath the wafer and collect the received radiationfrom above. The illumination light can be delivered by optical trains,fiber optics, or other directing elements.

[0048] III—The Property and/or Parameters by Which the Pixel Signaturesare Characterized

[0049] 1. In this system, the energy of the scattered light is the mainproperty that is being measured.

[0050] 2. Another property is the height of the surface. This ismeasured by the height measurement system.

[0051] 3. Other properties can also be used successfully for creation ofa signature. These are:

[0052] 3.1. The polarization of the received radiation, in P and Splanes. This is important, since there are many geometric locations atwhich the pattern on the wafer induces a well determined polarization,so that a correctly aligned polarizer would sense only particles.

[0053] 3.2. The phase of the received radiation.

[0054] 3.3. The wavelength of the received radiation, which can betested in various ways, e.g. by testing for fluorescence or by testingthe spectral response of a pixel.

[0055] With reference to the polarization of the received radiation, ithas been shown (see J. M. Elson, Multilayer coated optics: Guided wavecoupling and scattering by means of interface random roughness, J. Opt.Soc. Am. A12, pp. 729-742 (1995)), that the polarization direction fieldaround a patterned wafer surface, when illuminated with polarized light,exhibits a phenomenon whereby at certain collection angles thepolarization field is well defined. Thus, with a properly alignedpolarizer, the light scattered from the pattern will contribute almostzero energy at said angles. On the other hand, if there is a particle inthe illuminated spot, the light scattered from it is depolarized, andwill contribute significant energy at said angles, whereby the particlewill be clearly detected. It is not necessary to fix precisely thelocation and polarization direction of the detectors that will permit todetect a particle in this way: it suffices to provide a sufficientlyhigh number of detectors, each with a polarizing sheet in front. Theplurality of detectors ensures that some of them will get a responsethat will indicate the presence of a particle. When using polarizedlight, therefore, the method and apparatus of the invention need not bemodified as to the way of delivering the illuminating light anddetecting the light scattered from the wafer pixels, but a polarizershould be placed in front of each detector, no other change beingrequired in the apparatus, and the signal processing algorithms shouldbe modified to take into account the fact that the detectors whichgenerally capture low levels are those placed at the aforesaid angles inwhich the polarization field is well defined, so that if the lightcollected by those detectors has significant energy, the algorithmshould signal the presence of a particle.

[0056] IV—The Physical and Geometric Parameters of the Detection of theOptical Reaction of the Pixels

[0057] The optical reaction, and therefore the signature of the pixelsis defined by the light scattered by the pixels. The way in which it isdetected can vary widely. It is detected in a plurality of directions,which will be called, for descriptive purposes, “fixed directions”. Eachdirection is defined by a line from the pixel to a point of lightcollection. Therefore, the geometry of the scattered light detection isdefined by the disposition of the points of light collection. Saidpoints may be disposed e.g. in azimuthal symmetry on horizontalconcentric circles (“horizontal” meaning herein parallel to the wafersurface), or in elevational symmetry on vertical or slantedsemi-circles, or in a flat grid parallel to the wafer surface, or in asemi-spherical or other vault-like arrangement above the wafer.

[0058] In a preferred form of the invention, said signature is definedby an array of signals, each of which measures the intensity of thelight scattered by the pixel in a direction, and will be called herein“signature component”. The number of directions in which said intensityis measured, and therefore the number of signature components should besufficiently high for the signatures to characterize the correspondingpixels, as hereinafter better explained. The said signals are sampled ata given frequency “f”, which will be called “the sampling frequency”.The period of time between successive samplings, t=1/f, will be called“the sampling period”. The sampling frequency used in carrying out theinvention is preferably very high, in the order of millions of Herz,e.g. 11 Mhz. Each sample generates an array of digital signals, whichdefines the signature of the pixel that was illuminated by the beam atthe moment the sample was taken.

[0059] The term “scanning beam” is to be construed herein as meaning abeam that has a relative motion with respect to the wafer andsuccessively impinges on different points of the wafer. The inventioncomprises relative motions of any kinematic nature and produced by anymechanical means, as long as they cause the spot of the beam to moveover the wafer surface. By “spot of the beam” (also called “the beamfootprint”) is meant the area of the wafer illuminated by the beam atany moment, or in other words, the intersection of the beam with thesurface of the wafer. In view of the relationship between the wavelengthof the scanning beam and the dimensions of the elements of the waferpattern and of the foreign particles, the light scattered by the waferis diffracted.

[0060] The term “pixel”, as used in this specification and claims, meansthe area covered by the spot of the beam at the moment a sampling iscarried out, viz. the moment at which the digital signals, representingthe intensity of the light scattered by the wafer in the fixeddirections, are determined. Ideally, each pixel should border on thepixels adjacent to it, but this is not necessary for successfullycarrying out the invention. In practice, depending on the character andspeed of the relative motion of the scanning beam with respect to thewafer, on the area of spot size, and on the sampling frequency, adjacentpixels may overlap, so that each point of the wafer is examined morethan once, or, on the contrary, the adjacent pixels may be spaced fromone another, so that not all the points of the wafer will be examined.One or the other relationship between pixels may be chosen, and therelative motion of the scanning beam with respect to the wafer may bedetermined as desired, taking into account such parameter as theresulting amount of data and the speed of the operation.

[0061] In order to determine whether a signature has the characteristicsof a signature of a faultless or of a defective pixel, any criterionthat is adapted to the specific conditions in which invention is carriedout, and provides the desired type and degree of selection, can beadopted. The criterion may comprise a comparison between the controlledsignature and a master signature, or the definition of ranges ofacceptable parameters in which the parameters of the controlledsignature must be included, or the position of the controlled signaturein a statistics of signatures, and so on. A broadly suitable and simplemethod will be described hereinafter by way of example.

[0062] According to another aspect of the invention, at least one sourceof an irradiating beam, preferably a laser diode, is provided and ispreferably motionless, the controlled wafer is preferably rotated, morepreferably about its center, and is translated (viz. displaced parallelto itself along a straight or curved line), preferably by displacing itscenter in a line that lies in a plane perpendicular to its axis ofrotation, so as to move the spot of the beam over the surface of thewafer, and the light scattered by the wafer is collected in a pluralityof directions. These directions, in which the scattered light iscollected, will be called hereinafter “fixed directions”. The rotarymotion of the wafer has considerable advantages, in particular it iseasy to effect by mechanical means of conventional precision and permitsto achieve very high process velocities and therefore a very highthroughput, while having a small footprint. Although a rotational and atranslational motion of the wafer, the scanning beam being motionless,have been mentioned hereinbefore as preferable, it is the relativemotion of the beam with respect to the wafer that is the determiningfactor, and any manner of obtaining it is equally within the scope ofthe invention. Preferably, in each fixed direction the collected lightis transduced to an electric signal and this latter is converted to adigital signal—a pixel component—by sampling.

[0063] In a variant of the above aspect of the invention, a singlescanning beam is provided and the wafer is so moved that said beam scansthe entire surface of the wafer. A plurality of lasers, the spot sizesof which substantially overlap, are considered herein as producing asingle scanning beam.

[0064] In another variant, the surface of the wafer is partitioned intoa number of zones, a number of scanning beams (preferably equal to saidnumber of zones) is provided, each scanning beam being associated withone of said zones, the inspected wafer is so moved that each beam scansthe wafer zone associated with it, and the light produced by thescattering of each beam by the wafer surface is collected in a pluralityof fixed directions associated with said beam. Typically and preferably,said zones of the wafer, except the central one, which is circular, areannular, concentric rings having similar radial dimensions, and thewafer is rotated and is shifted approximately radially by an amountequal to said radial dimension of the rings. This variant of theinvention shortens the processing times, requires smaller motions of theapparatus elements and permits to define smaller pixels.

[0065] In a further preferred form, the process of the inventioncomprises the following steps:

[0066] 1—irradiating each wafer with one laser beam or with a pluralityof laser beams;

[0067] 2—causing a relative motion of each wafer with respect to saidbeam, if one laser beam is used, to cause said beam to scan the wafer,and if a plurality of laser beams is provided, to cause each beam toscan a zone of the wafer associated with it;

[0068] 3—sensing the light scattered by the wafer in a plurality offixed directions, if a single beam is provided, or in a number of suchpluralities associated each with a beam, if more than one beam isprovided;

[0069] 4—converting said scattered light, in each fixed direction, to anelectric signal;

[0070] 5—sampling said electric signal at a predetermined samplingfrequency, whereby to determine, at each sampling, an array of values,one value in each fixed direction, associated with a pixel of the wafer;

[0071] 6—considering each said array of significant values asconstituting a pixel signature;

[0072] 7—defining the conditions which must be satisfied by all thepixel signatures of a faultless wafer;

[0073] 8—determining whether the pixel signatures of each wafer meet thesaid conditions; and

[0074] 9—classifying the pixels which meet the said conditions, asacceptable pixels and the remaining pixels as “suspect”.

[0075] In an embodiment of the invention, a group of beams may be usedto scan a wafer by focusing them so that all have the same spot on thewafer surface, in this case, the scattered light produced by all thebeams will be collected in the same fixed directions.

[0076] Concurrently with the identification of the suspect pixels, theirlocation on the wafer is recorded to permit successive vector die-to-diecomparison. At each moment of the process, the position of the pixelsunder examination is identified in the machine coordinate system. Inthat system, the position of each pixel is defined by the angle by whichthe wafer support has rotated and by the distance of the pixel from thewafer center, or, as may be said, its radial position, which depends onthe displacement which the wafer center has undergone with respect tothe laser beam. Said angular and radial positions constitute the polarcoordinates of the pixels. The position of the pixel on the wafer, onthe other hand, is defined in the die coordinate system, in which apoint is identified by the index of the die it is in and the coordinatesof the point inside the die, with the axes parallel to the principaldirections of the die and the distances measured in microns. The way inwhich the die coordinates of a point are calculated will be describedhereinafter.

[0077] In a preferred form of the invention, the signatures of thepixels are transmitted, together with their coordinates, to a hardwarecomponent of the apparatus. By “hardware component” is meant herein anelectronic device having a specific task or a number of specific taskswhich can be selected as desired in each case. In general, the hardwarecomponent is a specially designed digital electronic device, the task ofwhich is to analyze the signals and make the preliminary selectionbetween signals that represent a valid pattern on the wafer and thosethat are suspected to arise from a contaminated spot. The signatures ofthe suspect pixels and their coordinates are transmitted further to asoftware component, which completes the die-to-die comparison. It willbe understood that, since the suspect pixels are only a small fractionof all the wafer's pixels, the information thus outputted by thehardware component is a small fraction of the information received byit.

[0078] An embodiment of the invention therefore comprises determiningthe position of the apparently defective pixels in the suspect wafers.Another embodiment of the invention comprises measuring the height ofthe pixels. Each type of wafer has a pattern having a given depth. Largeforeign particles often have a height, viz. a dimension perpendicular tothe wafer surface, in excess of said depth of the wafer pattern, andtherefore protrude from said pattern and their presence can be detectedby a height measurement.

[0079] The invention further comprises an apparatus for thedetermination of defects, particularly the presence of foreignparticles, in patterned, semiconductor wafers, which comprises:

[0080] a) a turn table for supporting a wafer and rotating it;

[0081] b) a light source and optics for generating at least one lightbeam and directing it onto the wafer;

[0082] c) means for shifting the spot of said beam relative to the wafercenter, preferably by shifting the axis of rotation of the wafer;

[0083] d) collection optics for collecting the light scattered by thewafer in a number of fixed directions;

[0084] e) photoelectric sensors for generating electric analog signalsrepresenting said scattered light;

[0085] e) A/D converter for sampling said analog signals at apredetermined frequency and converting them to successions of digitalcomponents defining pixel signatures;

[0086] f) means for determining the coordinates of each pixel;

[0087] g) a hardware filter for receiving the pixel signatures and theircoordinates and identifying the signatures that are not signatures offaultless pixels, viz. that are signatures of suspect pixels; and

[0088] h) a software algorithm for receiving from the filter thesignatures of suspect pixels, together with the corresponding pixelcoordinates, and carrying out a vector die-to-die comparison.

[0089] In a preferred embodiment of the apparatus according to theinvention, the light beam is a laser beam. In a more preferredembodiment, the means for generating a laser beam and the means forcollecting the laser light scattered by the wafer in a number of fixeddirections are associated, in the appropriate geometrical relationship,in a single structural unit, herein called “optical head”. An opticalhead generally comprises a single laser generator, but if it comprisesmore than one, the generators are so focused as to produce a singleillumination spot.

[0090] In said embodiment, therefore, the apparatus comprises:

[0091] a) a turn table for supporting a wafer and rotating the sameabout an axis of rotation that coincides with the geometric axis of thewafer;

[0092] b) means for translationally shifting the axis of rotation of thewafer;

[0093] c) at least one optical head;

[0094] d) photoelectric means for transducing the optical signalsgenerated in said optical head to electric analog signals;

[0095] e) A/D converter for sampling said electric analog signals at apredetermined frequency and converting them to successions of digitalcomponents defining pixel signatures;

[0096] g) a hardware filter for receiving the pixel signatures and theircoordinates and identifying the signatures that are not signatures offaultless pixels, viz. that are signatures of suspect pixels; and

[0097] h) a software algorithm for receiving from the filter thesignatures of suspect pixels, together with the corresponding pixelcoordinates, and carrying out a vector die-to-die comparison.

[0098] The optical head is, in itself, an object of the invention.

[0099] In an aspect of the invention, the apparatus comprises, incombination with mechanical means for supporting and rotating a wafer,optical means for substantially isotropically collecting the lightscattered by the wafer, and hardware means for taking into account anyangular displacement of the principal directions of the wafer dies withrespect to the wafer support plate. By “substantially isotropicallycollecting the scattered light” is meant collecting it at capture anglesthat are so many and densely distributed that an angular displacement ofthe optical collecting means will not significantly change the opticalsignals so collected. In other words, the optical collecting means willbehave approximately as if they were constituted by rings, set in planesperpendicular to the axis of the wafer rotation, uniformly sensing thescattered light at every point thereof. The means for taking intoaccount any angular displacement of the principal directions of thedies, with respect to the wafer support plate, comprises means fortransforming the optical signals actually received to the values theywould have if all the wafers were mounted on their support plate withtheir principal directions set in an invariable, predeterminedorientation.

[0100] The signature of any given pixel depends on certain operatingparameters, which must be specified and remain constant in any reductionto practice of the invention. The parameters comprise: a) thecharacteristics of the irradiating light, such as the type of lightsources, the number of such sources, the direction of the irradiatingbeam or beams, their wavelength, their energy fluence, the area of theirspot size, etc.; b) the fixed directions, viz. their number and theirorientation, both as azimuth and as elevation with respect to thesubstrate surface; c) the solid angle within which reflected light issensed by each sensor. Other parameters, referring to the mechanics ofthe invention, will become apparent later. If any of said parameters ischanged, the pixel signatures will change correspondingly. Therefore,said parameters must remain the same in any operation carried outaccording to or for the purpose of this invention. Generally, the largerthe number of fixed directions, the better the resolution of thescattered light and the completeness of the pixel signatures. Structuralconsiderations, on the other hand, prevent using an excessive number ofthem. It has been found that a satisfactory compromise between saidcontrasting factors is to use 16 or 32 fixed directions andcorresponding scattered light collectors. For simplicity ofillustration, in the following description, it will be assumed thatthere are two superimposed rings of fixed directions, each of whichcomprises 16 fixed directions. In each ring, the fixed directions areuniformly spaced in azimuth and have the same elevation angle. The tworings have different elevation angles. By “elevation angle” is meantherein the angle which the direction makes with the plane of the wafer.The plane of a wafer is defined as the plane of its upper surface. Theazimuthal and elevational angles are determined so that all fixeddirections intersect the plane of the wafer at the same point. Theaforesaid fixed direction configuration may also be described by sayingthat said directions lie on two conical surfaces having as their axisthe axis of the wafer and a common vertex, and that they are evenlyspaced on each conical surface.

[0101] The scattered light is preferably collected by at least oneoptical fiber bundle for each fixed direction, and transmitted tophotoelectric detectors, in each of which a continuous signal isgenerated. The terminals of each bundle, which lie on the fixeddirections, preferably abut on one another, so that each ring of opticalfiber terminals, lying on one of said conical surfaces, is continuous.It can be said that the optical fiber bundles are preferably“interlaced”. The photodetectors, which are conventional apparatus (anexample of which is OSD50, manufactured by Centronics), producecontinuous electric signals. The sampling of the continuous electricsignal produced by each photoelectric detector, can be carried out byapparatus known in the art and available on the market (e.g. AD9059RS,manufactured by Analog Modules) at frequencies of millions of Hz, sothat the number of pixels for which a signature is obtained is in theorder of millions per second, e.g. 11 Mpix/sec.

[0102] The scanning beam generally has an oblong spot size, e.g. havinga radial dimension (viz. a dimension parallel or approximately parallelto the wafer radius) between 5 and 15 microns and a tangential dimension(viz. a dimension perpendicular to the radial one) between 3 and 5microns.

BRIEF DESCRIPTION OF THE DRAWINGS

[0103] In the drawings:

[0104]FIG. 1 is a schematic illustration, in plane view, of the generalfeatures of an apparatus according to an embodiment of the invention;

[0105]FIG. 2 is a schematic illustration, in elevational view, of theapparatus of FIG. 1;

[0106]FIG. 3 is a schematic illustration, in elevational view, of anapparatus according to an embodiment of the invention;

[0107]FIG. 4 is a schematic plan view of another embodiment of theinvention;

[0108]FIG. 5 is a schematic plan view of a variant of the embodiment ofFIG. 4;

[0109]FIGS. 6a and 6 b are schematic vertical cross-sections of twoembodiments of optical head;

[0110]FIG. 7 is a plan view, from the bottom, of said optical head, at agreater scale;

[0111]FIG. 8 is a block diagram generally illustrating the phases of thescattered light processing according to an embodiment of the invention;

[0112]FIG. 9 is a block diagram generally illustrating the analogprocessing unit of an apparatus according to an embodiment of theinvention;

[0113]FIGS. 10, 11 and 12 an embodiment of the signal processing;

[0114]FIG. 13 illustrates the die coordinate system;

[0115] FIGS. 14(a), (b) and (c) and FIG. 15 schematically illustratealternative dispositions of the scattered radiation collectors;

[0116]FIGS. 16 and 17(a), (b) and (c) illustrate a method and apparatusfor height measurement; and

[0117]FIG. 18 is a conceptual flow chart exemplifying the vectordie-to-die comparison according to the preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0118]FIGS. 1 and 2 schematically represent an apparatus according to anembodiment of the invention. Numeral 10 indicates a wafer that is beinginspected. The apparatus used for the inspection comprises a stagehaving a wafer support. The wafer is placed on said support, which inthis embodiment is a support plate 11, which is rotated about shaft 12by mechanical means, not shown as being conventional. A laser source isshown at 13 in its central position, above the axis of shaft 12.However, more than one source could be provided and any source could beplaced at an angle to the axis of shaft 12, to provide the requiredillumination of the wafer, depending on the type of wafer underinspection. In FIG. 2 one such additional laser source is shown, by wayof illustration, oriented at an angle β from the plane of the wafer.Mechanical means, not shown as being conventional, translate the shaft12, viz. shift it, while maintaining it parallel to itself, so that anypoint thereof moves in a straight or curved line that lies in a planeperpendicular to the axis of the shaft. Consequently the wafer 10 isalso translated parallel to itself so that its center of the wafer isshifted in a plane perpendicular to the axis of the shaft. By“translatory motion” is meant any motion of a body in which the bodydoes not rotate, but is displaced without rotation along any straight orcurved line. The translational displacement of shaft 12 and wafer 10 isrectilinear, but it need not be: if desired, it could follow a curvedpath.

[0119] Additionally, the laser source or sources may not be stationaryand the corresponding spot or spots on the wafer surface may move in away similar to the motion of a needle on a phonograph disk, viz. swingalong an arc of circle passing through the center of the wafer.Consequently, since the laser source 13 remains stationary, the spot ofthe scanning beam is displaced on the wafer from the periphery of thewafer to its center and/or vice versa, and, possibly but notnecessarily, along or approximately along a radius of the wafer 10.Preferably, the wafer is rotated at a V_(r) of 5000 rpm and displacedradially at a speed L of 0.01-0.5 cm/sec.

[0120] The light of beam 13 is scattered in the fixed directions. Thefixed directions may be azimuthally or elevationally distinct. In theembodiment illustrated, they are azimuthally distinct and arranged in 2rings of 16 each. In each ring, the said directions are symmetricallyarranged about the wafer and slanted at an elevation angle α₁ for thelower ring and α₂ for the upper ring, from the plane of the wafer. Theangle α₁ is selected from 8 to 15 degrees, and the angle α₂ from 15 to30 degrees. This arrangement, however, is merely an example, and can bevaried as desired, as will be better explained later.

[0121] In each fixed direction, the scattered light is collected by anoptical fiber terminal—15 in the lower ring and 16 in the upper ring.Preferably, the generator of the beam 13 and the optical fiber terminalsare structurally associated in an optical head. Each optical fibertransfers the collected light to a photodetector—17 in the lower ringand 18 in the upper ring.

[0122] Each photodetector outputs an electric signal, which istransmitted to an electronic circuit, not shown in the drawing, whichsamples the electric signals and outputs, for each fixed direction, adigital signal corresponding to the intensity of the light collected bythe corresponding optical fiber. The sampling frequency f may be, e.g.,11 MHz. The spot size of the scanning laser beam, in this example, hasan approximately elliptical shape, with a longer diameter of 15 μm and ashorter diameter of 5 μm.

[0123]FIG. 3 is a further schematic illustration, in elevational view,of a machine according to an embodiment of the invention. The machinecomprises a frame 20, on which a mechanical assembly, generallyindicated at 21, is supported. The mechanical assembly comprises a motorassembly 23, which rotates a plate 22 that supports the wafer. Numeral24 schematically indicates means for translationally displacing saidmotor and plate. Numeral 25 generally indicates a scanning system, whichactuates a scanning head 26, containing the laser sources, the opticalfibers for collecting the scattered light, and the photoelectricdetectors. Block 27 schematically indicates the electronic components ofthe machine, which receive the output of the photodetectors throughconnections (not shown) and process it as herein described.

[0124] It should be noted that, besides the aforementioned rotary andtranslational motions of the wafer supporting plate, which occur duringthe scanning of the wafers, different motions are required for carryingout the stages of loading and unloading the wafers. Additionally, anautofocusing mechanism is preferably provided for focusing theilluminating beam, and such mechanisms (too) are well known to personsskilled in the art and need not be described

[0125]FIG. 4 is a plan view schematically illustrating anotherembodiment of the invention, which comprises a plurality of opticalheads. The wafer 30 is ideally divided into a number of zonesconstituted by concentric rings and a central circle. Only three ringzones—31, 32 and 33—in addition to central circular zone 34, are shownin the drawings for simplicity of illustration, but in practice theremay be more. An optical head, comprising a scanning beam and an array ofoptical fiber sensors in the fixed directions, is provided for eachzone. In each fixed direction the scattered light is collected by anoptical fiber sensor. In this embodiment, the scanning beam and theoptical fiber sensor of each zone are mounted on a common support, todefine an optical head, hereinafter described. The four optical heads,which are identical, are indicated by numerals 35, 36, 37 and 38. InFIG. 4 the optical heads are shown as being one for each zone andsuccessively aligned along a radius of the wafer, but this is merely aschematic illustration. The heads may not be aligned along a radius,but, for example, may be staggered and partially overlap, so that somecircles drawn on the wafer surface may cross more than one head. Such anarrangement is schematically indicated in FIG. 5, wherein a plurality ofheads, schematically indicated at 39, are staggered generally along aradius of a wafer 30, to cover an equal number of zones not indicated inthe figure. Said arrangement particularly applies to optical heads thatcomprises CCD detectors, that will be described hereinafter withreference to FIG. 15.

[0126] The purpose of these embodiments is to cause a plurality ofpixels to be illuminated and checked concurrently, whereby the processis accelerated and the throughput increased; and further, to limit thetranslational displacement of the wafer to a fraction of what it wouldbe if a single optical head were to scan the whole wafer, simplifyingthe mechanics of the machine and reducing its footprint. Saidtranslational displacement, as in other embodiments of the invention,need not be radial, but may follow a more a differently directedstraight path or a non-straight path, as may be more convenient in viewof the mechanics of the apparatus. Any disposition of optical headsand/or wafer translation that will serve the purpose of a completescanning by convenient mechanical and optical means can be adopted.

[0127]FIG. 6a is a vertical cross-section of one of the optical heads,of which FIG. 7 is a plan view, from the bottom, at a larger scale. Itis assumed to represent one of the optical heads 35 (or 39) but couldrepresent any other optical head. It could also represent the opticalhead of an apparatus which includes only one such head, the wafer beingdisplaced in such a way that the single head scans its entire surface.Head 35 comprises a base 40, which has at its bottom, a central recessor cavity 41 that is arc-shaped, e.g. approximately semi-spherical, andhas a bottom, viz. its opening, that will be parallel to the plane ofwafer when the head is used. Base 40 is mounted in a case 45, supportedin the machine in any convenient way, not illustrated. In base 40 aremounted the laser source 42 and two circular arrays of optical fibers 43and 44, disposed one above the other at different angles so as toconverge onto the center of recess 41, where the pixel that is beingexamined is located. It is seen in FIG. 7 that the terminals of saidoptical fibers, one of which is indicated by numeral 47, whichconstitute the intersection of said fibers with the surface of cavity41, are adjacent to one another, so that each array of said fibers formsa continuous circle at the surface of cavity 41. While two fibers 43 andtwo fibers 44 are shown in the cross-section of FIG. 6a, each arraypreferably comprises, in this embodiment, 16 optical fibers, which aregathered, in this embodiment, into two bundles 46 for connection tophotodetectors, not shown. Further, a plurality of laser sources, e. g.placed at different angles to the wafer plane, and more than twocircular arrays of optical fibers could be comprised in an optical head.No matter how many rings of fiber terminals are provided, said terminalsare preferably disposed in each ring at uniform angular distances fromone another and are so slanted that all of their axes pass through acommon point, which is the center of the bottom of cavity 41 and will bethe center of the portion of the wafer surface exposed by said cavity,when the optical head is superimposed to a wafer surface to carry outthe process of the invention.

[0128]FIG. 6b schematically illustrates in vertical cross-section, at alarger scale, the optical components of another embodiment of opticalhead. Said head comprises three laser sources 42 a, 42 b and 42 c, thefirst oriented perpendicularly to the bottom of cavity 41 and the othersat different slants thereto, to provide an improved illumination. Twocircular arrays of optical fibers 43 and 44 are provided in this head aswell, and their terminals 48 and 49 form two circles about cavity 41. Itshould be appreciated that the laser source 42 can be situated remotelyfrom the optical head 35, and that the laser beam can be transmitted tothe head using, for example, other optics similar to collection opticfibers 43 and 44.

[0129]FIG. 8 is a block diagram generally illustrating the phases of theprocess by which the scattered light from a wafer surface is processedaccording to an embodiment of the invention. The process is illustratedwith respect to an optical head, as hereinbefore illustrated. If theapparatus of the invention comprises a plurality of optical heads, thesame operations are carried out with respect to each of them. If theoptical components of the apparatus are not combined in an optical head,and no matter how they are combined, what will be said about an opticalhead will apply to the processing of the light scattered by the wafersurface is carried out in the stages shown in FIG. 8. It is assumed forillustrative purposes only that the optical head unit 50 includes anillumination apparatus, two rings with 16 optical fibers each, whichdetect the light scattered in as many directions (which constitute thefixed directions), and any supporting subsystems that it is desired tointroduce, such as auto-focus mechanism, lenses, etc. Said opticalcomponents operate as hereinbefore described.

[0130] The analog processor unit 51 is responsible for detecting theoptical signals from the optical fibers, transducing the signals intoelectric pulses, amplifying said electric signals, applying a correctioncomputation to make sure that all the detectors (32 in this example) areproperly calibrated with respect to the inspected surface and withrespect to each other, and finally converting each electric analogsignal into a digital signal, preferably with 8 bits.

[0131] That is, each sample or pixel signature component is a digitalsignal constituted by a word having a sufficient number of bits toprovide adequate information. E.g., the word may be composed of 8 bits,providing 256 levels of scattered light intensity. Each ring of thephotodetectors will then contribute 16×8=128 bits with a frequency “f”,which, in this embodiment, is assumed to be 11 Mhz.

[0132] The digital signals are transferred as output to a signalprocessor unit 52. The resulting pixel signature outputs must beevaluated according to a predetermined criterion. While, within thescope of this invention, many criteria and corresponding algorithmscould be used, according to cases and to the choice of the expertperson, a simple criterion will later be described by way ofillustration.

[0133] The signal processor unit 52 is responsible for the first stageof the data reduction. It receives the input signals (32 in thisexample) from the analog processor unit, at the clock rate of the system(e. g. 11 MHz). It outputs only a small percentage of these data to thenext stage. The signal processor unit is a custom designed electronicsubunit that can handle very wide input data at a high rate. It is alsocapable of employing several different reduction algorithms, switchingbetween them as the application requires. The pixels, the data of whichare transmitted out of the signal processor, are henceforth termed“suspect pixels” or “suspects”. The signal processor is designed totransmit a small fraction of the inspected pixels to the following unit.This data is communicated via a FIFO bank (to coordinate thecommunication rates), through a standard bus, such as a PCI bus, to themain CPU of the system.

[0134] The defect detection unit 53 is a software module running on themain CPU of the system. It receives suspect pixel data from the signalprocessor unit. Its responsibility is to separate the valid pixels fromthe defective ones and output the defect list as the final product ofthe system.

[0135] Considering now the components of the block diagram of FIG. 8individually, the preferred structure of the Optical Head Unit 50 hadbeen discussed hereinbefore.

[0136] The internal structure of the Analog Processor Unit 51 of thisexample is schematically illustrated in FIG. 9. The unit is composed ofa number of identical channels, one for each optical fiber, three ofwhich are symbolically indicated in perspective relationship. Eachchannel (out of the 32 of this example) comprises a detector 55 thattransduces the light signal into electric current. The electric signalis then is amplified in a preset amount in a preamplifier 56. Saidsignal is then further amplified with an amplifier 57 that has variablegain and offset. This allows the system to adapt to varying substrates,illumination angles and parameters such as wavelength, intensity, etc.,and also allows to calibrate the several channel signals to respondequally. This insures the isotropy of the whole optical channel, so thatif the same pixel is observed from varying angles (for example, afterrotation of the wafer), the same signature will be obtained from all thesignals. It is also a prerequisite for all subsequent treatment of thesignals. The last block of this Analog Processor Unit is an A/Dconverter 58 that performs analog to digital conversion preferably at 8bits. Thus, in this example, the output of Analog Processor Unit 51 is32 signals, each with 8 bits, at a system clock of 11 Mhz.

[0137] The Signal Processor Unit 52 is a specially designed digitalelectronic device, the task of which is to analyze the signals and makethe preliminary selection between signals that represent a valid patternon the wafer, and those that are suspected to arise from a contaminatedspot. Such suspected signals are passed on to the next analysis step.Preferably, this unit should be reconfigurable to apply variousalgorithms for the discrimination between valid and suspect pixels,changing the algorithm according to the demands of the application. Theoperation of this unit will be explained in algorithmic building blocks.The implementation of the algorithms as a hardware device is well knownto electronic engineers skilled in the art of designing modern digitalsignal processing boards, especially of the kind that is based on FPGA(Field Programmable Gate Array) and DSP (Digital Signal Processor)technology. The details of this implementation are not part of theinvention and need not be discussed herein.

[0138] While a variety of algorithms may be devised and used by skilledpersons, a specific algorithm will be described by way of example, withreference to FIG. 10.

[0139] The digital signals from Analog Processor Unit 51 (32 in thisexample) firstly enter into a bump detector 61, each signalindependently of the others. The bump 62 is graphically illustrated inFIG. 11. It is detected with the following operator, that requires threeparameters: total width 63, central width 64 and a ratio threshold 65between the brightness peak 66 of the pixel currently under control andthe highest brightness peak 67 in the filter's domain that is outside acentral patch. By “width” is meant the number of data taken at one time.The values of the two peaks are compared in threshold comparator 69 todetermine their ratio. If the determined brightness ratio exceeds somethreshold, predetermined on the basis of experience, the pixel isretained in the signal, otherwise, it is zeroed. This is done for eachof the 32 signals, and the result is again a set of 32 signals. The mostcommon parameters for this operator have been found to be: filterwidth=11 pixels, central width=5 pixels and ratio threshold=1.3.

[0140] If at least one of the 32 signals produced by a pixel has a highbump, viz. its brightness ratio is above the threshold, all 32 signalsrelative to said pixel are passed on to this estimator for thestatistical evaluation. Therefore, in this embodiment, and optionally ingeneral, bump detection is a first preliminary stage filter performedprior to the vector die-to-die comparison. However, the number ofsignals relative to each pixel that have a brightness ratio above thethreshold (the output of bump detection) gives an indication as towhether the pixel is likely to be considered suspect. For instance, asingle signal having a high bump may be due to the wafer pattern,whereas a high number of such signals is probably due to a particle. Thedecay rate estimator 68 (see FIG. 10) analyzes the 32 signals relativeto each pixel (the output of bump detection) and provides somestatistical indicator of their values. Therefore, in this embodiment,and optionally in general, decay rate estimation is a second preliminarystage of the vector die-to-die comparison, successive to bump detection.The algorithm uses three parameters: central value percentile p, widthfactor w, and threshold s. The computation is as follows: Sort the 32values. Pick the p percentile value. Sum all values that are between p*wand p/w. The sum is sent to a threshold comparator. The result of thecomparator is 1 or 0, according to whether the sum is greater or smallerthan s. Common values have been found to be p=w=0.5. The value of s isvariable, and has to be empirically determined. This procedure isillustrated by an example in FIG. 12, wherein numeral 70 indicates acurve which interpolates all the percentile values. The area 71 (markedin black) under curve 70, between twice the center value and one halfthe center value, is compared to an empirical threshold.

[0141] Whereas the input to the signal processor unit is synchronousdata at the system's clock, the frequency of the output is on theaverage 2-4 orders of magnitude smaller. A standard interface with somememory is designed into the system to handle potential peaks ofactivity, and to push the data down a PCI bus to the host computer (e.g. Pentium II by Intel Corporation Limited). Each output suspect featureis built of coordinate data and type data. The coordinate data isprovided by the mechano-electronic subsystem in polar coordinates (ρ andθ) and can be translated, using the registration transformationdescribed herein, to wafer coordinates. The type data is in the case ofthe algorithm detailed above the output of the decay rate estimator (thenumber that was sent to the threshold comparator). This is an indicationof the strength of the detection, or, in other words, of the detectioncertainty.

[0142] The Defect Detection Unit 53 is a software module whose job is toreceive the data relative to all the suspect pixels from the previousstage, and find out which of them represent real defects, viz. to carryout the vector die-to-die comparison (VDDC), which follow thepreliminary stages of bump detection and decay rate estimation, if thesehave been carried out. The VDDC operation comprises the following:

[0143] 1. Transforming the polar coordinates of the machine to theCartesian coordinates of the die coordinate system.

[0144] 2. Deriving from the coordinates that define the suspect pixels'location in the machine coordinate system the coordinates that definesaid location in the die coordinate system.

[0145] 3. Marking in the die coordinate system the position of allsuspect pixels.

[0146] 4. Discriminating between suspect pixels that are due to thewafer pattern, and therefore do not represent a defect, and suspectpixels that are not due to it and therefore do represent a defect, inparticular those deriving from foreign particles.

[0147]FIG. 13 illustrates how the coordinates that define the suspectpixels' location in the die coordinate system are derived from theircoordinates in the machine coordinate system. When a new wafer 100 isplaced, generally by means of a robot, on the rotary plate of theinspection apparatus of the invention, the orientation of the wafer isunknown and it may be miscentered by up to 1 mm. It is important for thelater parts of the inspection procedure, and of course for the output ofthe list of defective pixels, to be able to describe the location ofeach pixel on the wafer with the wafer's natural frame of coordinates.The wafer is composed of many identical dies 101, each one of which isdestined to constitute (if not faulty) a semiconductor device or part ofsuch a device, such as a CPU or a memory chip. The dies are separated bythe scribe lines, along which the wafer will be cut when ready, whichform two perpendicular families of lines, that can be called, forconvenience of illustration, avenues 102 and streets 103. Avenues andstreets, collectively, can be called “the principal directions of thedie”. The avenues are considered as oriented from “south” to “north” andthe streets from “west” to “east”. At one point on the circumference ofthe wafer there is a small recess, called a notch 104, which defines thewafer's “south”.

[0148] The procedure of defining the wafer map and registering it withreference to the machine coordinate system, is carried out as follows:

[0149] 1. A short pre-scan is carried out, typically by rotating themachine plate by 100 turns, spanning about 20 mm width of the externalcircumference of the wafer.

[0150] 2. The notch is detected by its known, typical signature.

[0151] 3. Streets and avenue pixels are detected by their specificsignatures, which are determined by their being mirror-like in most oftheir area.

[0152] 4. The detected notch and street and avenue pixels aretransmitted to the CPU that controls the procedure.

[0153] 5. A registration algorithm receives the said input and computesthe angle of rotation of the wafer's coordinate system with respect tothe machine's coordinate system, and also the locations of the streetsand avenues.

[0154] This allows a map of the wafer to be constructed (when a newwafer is introduced) or a registration transformation to be computed (ifthe map is already known). If the wafer is a bare wafer, then only thenotch can be detected and a registration transformation of loweraccuracy can be computed, using the location of the notch. Suchregistration transformation can be carried out by means of knownalgorithms, e.g. by the randomized Hough transform technique—see L. Xu,E. Oja and P. Kultanen, A new curve detection method, Randomized HoughTransform (RHT), Pattern Recognition Letters, vol. 11. no. 5, May 1990,pp. 331-338, Elsevier Science Publishers B.V. (North-Holland—or by otheralgorithms easily devised by expert persons.

[0155] The inventive vector die-to-die comparison (VDDC) will now bedescribed. Conceptually, unlike the conventional die-to-die comparison,wherein each (x_(i), y_(i)) location on a die is compared to thecorresponding location on the preceding and following die, the VDDC“stacks” all the dies and checks to see whether a suspect (x₀, y₀)location appears in a corresponding location in more than one die. Theentire operation, including the coordinates transformation, can bebetter understood with reference to FIG. 18. FIG. 18 depicts a defectmap in the form of a wafer, 80, having a plurality of dies 82 thereon,and having “suspected” defect locations, each marked with an “x”.

[0156] In the apparatus according to the preferred embodiment, thesuspected locations are provided in polar coordinates, i.e., (r_(j),θ_(i)) pairs. Therefore, the suspected locations' coordinates are firsttransformed to the Cartesian coordinates of the wafer, i.e. (x_(j),y_(i)) pairs, and thence to the cartesian coordinates of thecorresponding dies, i.e. (x_(kj), y_(kj)) pairs (k standing for the dieand j standing for the coordinate within the die). Once the (x_(kj),y_(kj)) pairs of all the suspected locations within the wafer have beenobtained, the dies are “stacked” to see whether any suspect location(x_(kj), y_(kj)) appears in more than one die. Here, much informationabout the suspect locations can be obtained. For instance, if aparticular (x_(kj), y_(kj)) pair appears in all the dies, it is likelyto be a feature of the die structure and not a defect.

[0157] Also, as is known in the art, the patterns on the wafers arecreated by a process called photolithography, which uses reticles havingthe desired pattern thereupon. It is known to use reticles which have,for example, a multiple of four die patterns thereupon. Thus, if duringthe VDDC it is determined that a particular (x_(kj), y_(kj)) pairappears in every fourth die, it is likely that the defect has beentransferred from a defective reticle. Thus, this information can be usedto set thresholds and other filtering mechanisms for the VDDC.

[0158] As can be seen from the above, using the VDDC discrimination isnow effected between two types of suspect data:

[0159] a) Suspect data that are actually produced by the wafer pattern,but appear as if they were due to the presence of particles (andtherefore are detected as indicating such presence). These will appearin many or all of the dies. Thus, in die coordinates, one will seenumerous appearances of suspects at the same location. All the suspectsthat appear at that location are discarded. It may be advantageous tofilter the data in some way before the VDDC, for example by utilizing amethod that recognizes that a group of points form together somespecific geometric configuration, for example line segments. This groupcan then be considered as a legitimate pattern of the wafer and filteredout of the set of suspect points. This allows the VDDC to operate on asmaller set of isolated points and thereby to achieve betterperformance.

[0160] b) Suspect data that are produced by real contamination byparticles. These appear essentially only once on the die map.

[0161] The detailed construction of software that implements thisalgorithm is a routine task for a skilled algorithm designer.

[0162] It should be appreciated, of course, that rather than using theinventive VDDC, one may choose to perform a conventional die-to-die orcell-to-cell comparison. Even if such an approach is taken, theinventive system reduces processing time, since, unlike conventionaldie-to-die systems wherein all the pixels are compared to their nearneighbors, only the suspect pixels flagged by the Signal Processor Unit52 need to be compared to their near neighbors. Of course, in such acase, each time the Signal Processor Unit 52 flags a suspect pixel, thenear neighbor pixels need to be stored in the memory for the die-to-diecomparison.

[0163] FIGS. 14(a), (b) and (c) schematically illustrate the dispositionof scattered radiation detectors above the wafer surface and notperipherally, as in the previously described embodiments. The fixeddirections, therefore, are elevationally and not azimuthally spaced. Thewafer is indicated at 70. In FIG. 14(a) only one semicircular detectorring 71 is shown, along which detectors 72 are disposed. Such a ringwill detect radiation scattered on a plane perpendicular to the waferplane, at different elevational angles on said plane. In FIG. 14(b)several semicircular rings 73 of detectors are provided, in any desirednumber, though only three are shown for convenience of illustration inthe drawing. The rings are on different planes, differently slanted withrespect to the wafer plane. If there is an odd number or rings, thecentral one will be on a plane perpendicular to the wafer plane. FIG.14(c) shows two rings 74 of detectors, disposed on two planesperpendicular to the wafer plane and perpendicular to each other. Inthis case too, any desired number of detector rings could be provided.It is clear that the geometric arrangement of the detectors can bechanged as desired.

[0164] In a further embodiment of the invention, schematicallyillustrated in FIGS. 5 and 15, pixel signatures are determined bycomponents that are determined by measuring elevation angles scatteringrather than azimuthal angles scattering signature. In this case, it ispossible and preferable to use line CCD detectors and line laser diodebars placed as a fan. Thus FIG. 15 shows a wafer 110 with a set oflinear CCD elements 111. For instance, there can be used CCDs with 1000detectors each, arranged as a fan with 10 units. One of these elementscan be replaced with a laser diode bar. This provides the requiredillumination, together with the versatility needed to set theillumination angle to suit each particular case. With 1000 detectors,each capturing the energy from a pixel with radial dimension of 15microns, one would cover 15 mm. of the wafer's radius, that is, about10%. Therefore an apparatus according to the invention with e.g. 10static heads could be used in place of an apparatus with one head movingradially across the wafer.

[0165] The depth measurement is optional and, when effected, can becarried out by devices known in the art as to themselves, although neverbefore applied to the testing of semiconductor wafers. Such devices areused, e.g. as autofocusing mechanisms in the Compact Disk art. See forexample H. D. Wolpert, “Autoranging/Autofocus—A Survey of Systems”,Photonics-Spectra, June p.65, August p.127, September pp. 133-42, Vol.21, Nos. 8 and 9 (1987). Schematically, they may be constituted andoperate as illustrated by way of example in FIGS. 16 and 17. In FIG. 16,50 designated a portion of a patterned wafer surface, on which a largeparticle may have been deposited. A laser diode 52 emits a beam whichenters a diffraction grating 53, which converts the beam into a centralpeak plus side peaks. The resulting three beams go though a polarizingbeam splitter 54, which only transmits polarization parallel to a plane,which in this example is assumed to be the plane of the drawing. Theemerging, polarized light is collimated by collimator 55. The collimatedlight goes through a ¼ wave plate 56. This converts it into circularlypolarized light. The circularly polarized light is then focused onto thewafer through objective lens 57. The light reflected by the wafer goesback into the objective lens 57 and then passes once again through the ¼plate. Since it is going in the reverse direction, it is polarizedperpendicularly to the original beam, viz. perpendicularly, in thisexample, to he plane of the drawing. When said light hits the beamsplitter 54 once again, it is reflected through a focusing lens 58 and acylindrical lens 59 and is imaged on a photodetector array 60. If theobjective lens is closer to the reflecting area of the wafer than thefocal plane 61 of the objective lens 57, an elliptical image is createdon the photodetector array 60, as shown in FIG., 17(a). If it is fartheraway than said focal plane, another elliptical image is created,perpendicular to the first one, as shown in FIG. 17(b). If thereflecting area of the wafer is at the focal length of the objectivelens, the cylindrical lens does not affect then image, which iscircular, as shown in FIG. 17(c). Therefore, if the pattern of a waferproduces a circular image, due to the lands of the pattern, viz. theplane of the wafer, being at the focal plane of the objective lens, andwhen a given pixel is illuminated an elliptical image is formed, thiswill indicate the presence of a particle the size of which cause it toproject above the said pattern lands. The displacement signal of theobjective lens, required to reestablish a circular image, can give ameasure of the amount by which the particle projects above the plane ofthe wafer.

[0166] It should be noted that, while the invention has been describedand illustrated on the assumption that the surface to be analyzed is theupper surface of a body, in particular of a wafer, and therefore theirradiating beam is directed downwardly onto it, the supporting shaft islocated below it, and in general all parts of the apparatus conform tothis geometric orientation, the apparatus could be differently oriented,and, e.g., overturned, so that that the surface to be analyzed be thelower surface of a body, in particular of a wafer, with all theattending structural consequences.

[0167] While examples of the method and apparatus of the invention havebeen described by way of illustration, it will be apparent that theinvention may be carried out with many variations, modifications andadaptations by persons skilled in the art, without departing from itsspirit or exceeding the scope of the claims.

1. Method for the analysis of surfaces, particularly for the detectionof defects on semiconductor wafers, which comprises checking individualpixels of the surface under control, and detecting suspected pixels bycollecting the signature of each pixel, defined by the way in which thepixel responds to the light of a scanning beam, and determining whethersaid signature has the characteristics of a signature of a faultless orof a pixel that is defective or suspect to be defective.
 2. Methodaccording to claim 1, comprising analyzing the signature of each pixelto determine the presence of foreign particles.
 3. Method according toclaim 1, wherein a pixel signature is defined by an array of signaturecomponents, each of which is a signal which corresponds to the intensityof the light scattered by the pixel in a fixed direction.
 4. Methodaccording to claim 1, comprising detecting defective or suspect pixelsby a method chosen from among the group consisting of comparing thepixel signature to a master signature, comparing parameters of the pixelsignature to ranges of acceptable parameters, or determining theposition of the pixel signature in a statistics of such signatures. 5.Method for the analysis of patterned, semiconductor wafer having aplurality of dies thereon, which comprises providing at least one sourceof scanning beam, causing the beam and the wafer to move relatively toone another, sampling the light scattered by the wafer in a plurality offixed directions, so as to obtain a plurality of pixels, each of saidpixels having polar coordinates associated therewith, and transformingthe polar coordinates of each suspected pixel to cartesian coordinatesof the corresponding die.
 6. Method according to claim 5, wherein thesurface of the wafer is ideally divided into a number of zones, scanningbeams are provided in a number equal to said number of zones, eachscanning beam being associated with one of said zones, and the wafer isso moved that each beam scans the wafer zone associated with it, and thelight produced by the response of the wafer surface to each beam iscollected in a plurality of fixed directions associated with said beam.7. Method according to claim 6, wherein the zones of the wafer areannular, concentric rings having the same radial dimension, and thewafer is rotated about its center and is shifted radially by an amountequal to said radial dimension of the rings.
 8. Method for the analysisof patterned, semiconductor wafers, which comprises the steps of:irradiating each wafer with a laser beam; causing a relative motion ofeach wafer with respect to said beam, to cause said beam to scan thewafer; sensing the light scattered by the wafer in an array of fixeddirections; converting said scattered light, in each fixed direction, toan electric signal; sampling said electric signal at a predeterminedsampling frequency, whereby to determine, at each sampling, an array ofvalues, one value in each fixed direction, associated with a pixel ofthe wafer; considering each said array of values as constituting a pixelsignature; defining the conditions which must be satisfied by all thepixel signatures of a faultless wafer; determining whether the pixelsignatures of each wafer meet the said conditions; and classifying thepixels which meet the said conditions, as acceptable pixels andclassifying the remaining pixels as “suspect”.
 9. Method according toclaim 8, wherein, after defining the signature of each pixel, at leastone signal of each signature is evaluated, and, based on saidevaluation, a number of signatures is excluded from further processing.10. Method for the analysis of patterned, semiconductor wafers, whichcomprises the steps of: dividing surface of each wafer into a number ofzones; irradiating each wafer with a number of laser beams, eachassociated with one of said zones; causing a relative motion of eachwafer with respect to said beams to cause said beams to scan the zonesof the wafer; sensing the light scattered by the wafer in a number ofarrays of fixed directions, each associated with a beam; converting saidscattered light, in each fixed direction, to an electrical signal;sampling said electric signal at a predetermined sampling frequency,whereby to determine, at each sampling, an array of values, one value ineach fixed direction, associated with a pixel of the wafer; consideringeach said array of values as constituting a pixel signature; definingthe conditions which must be satisfied by all the pixel signatures of afaultless wafer; determining whether the pixel signatures of each wafermeet the said conditions; and classifying the pixels which meet the saidconditions, as acceptable pixels and classifying the remaining pixels assuspect.
 11. Method according to claim, 1, further comprising subjectingeach wafer comprising suspect pixels to vector die-to-die comparison.12. Method according to claim 1, further comprising measuring the heightof the pixels to detect large foreign particles.
 13. Method ofdie-to-die comparison of a plurality of dies in a semconductor wafer,comprising: obtaining die coordinates of suspect pixels in each of saiddies; for each of said suspect pixels determining whether anothersuspect pixel exists in similar coordinates in other dies.
 14. Method ofdetecting defects in a semiconductor wafer, which comprises: carryingout a step of bump detection to identify suspect pixels; obtaining diecoordinates of suspect pixels in each of said dies; for each of saidsuspect pixels determining whether another suspect pixel exists insimilar coordinates in other dies.
 15. Method of detecting defects in asemiconductor wafer, which comprises: carrying out a step of decay rateestimation to identify suspect pixels; obtaining die coordinates ofsuspect pixels in each of said dies; for each of said suspect pixelsdetermining whether another suspect pixel exists in similar coordinatesin other dies.
 16. Apparatus for the determination of defects,particularly the presence of foreign particles, in patterned,semiconductor wafers, which comprises: a) a stage having a support; b) alaser source and optics generating a laser beam and directing it ontothe wafer; c) collecting optics for collecting the laser light scatteredby the wafer in a number of fixed directions; d) photoelectric sensorsfor generating electric analog signals representing said scatteredlight; e) A/D converter for sampling said analog signals at apredetermined frequency and converting them to successions of digitalcomponents defining pixel signatures; f) first selection systemreceiving the pixel signatures and their coordinates and identifying thesignatures that are signatures of suspect pixels; and h) secondselection system receiving from said first selection system thesignatures of suspect pixels, together with the corresponding pixelcoordinates, and verifying whether each suspect pixel is indeed adefect.
 17. Apparatus according to claim 16, wherein the stage comprisesa turn table and scanning is accomplished by shifting the axis ofrotation of the turn table.
 18. Apparatus for the determination ofdefects in patterned, semiconductor wafers, which comprises: a) a stagehaving a wafer support; b) a laser source generating a laser beam; c) atleast one optical head designed to transmit the laser beam onto thewafer, and having a plurality of collection fiber optics arrangedtherein; d) photoelectric sensors transducing the light collected by thecollecting fiber optics to electric analog signals; e) A/D converter forsampling said electric analog signals at a predetermined frequency andconverting them to successions of digital components defining pixelsignatures; g) selection hardware receiving the pixel signatures andtheir coordinates and identifying the signatures of suspect pixels; andh) microprocessor responsive to selection software to receive from saidselection hardware the signatures of suspect pixels, together with thecorresponding pixel coordinates, and evaluating said suspect pixels tosingle out false alarms.
 19. Apparatus according to claim 18, whereinthe optical head comprises at least one laser generator and wherein saidcollection fiber optics comprises two superimposed rings of opticalfibers for collecting the light scattered by the wafer.
 20. Apparatusaccording to claim 19, wherein each ring comprises 16 optical fibers,uniformly spaced in azimuth and having the same elevation angle, the tworings having different elevation angles,
 21. Apparatus according toclaim 19, wherein the laser generator produces an oblong spot size. 22.Apparatus according to claim 18, wherein the selection hardware is acustom designed electronic circuit comprising a bump detector, a decayrate estimator and a threshold comparator.
 23. Apparatus according toclaim 18, further comprising means for measuring the depth of the waferpixels.
 24. Optical head, comprising a cavity exposing the surface undercontrol, at least one laser source and a plurality of optical fibershaving terminals symmetrically disposed about said cavity.
 25. Opticalhead according to claim 24, wherein the optical fiber terminals aredisposed in a plurality of superimposed rings, are evenly spaced in eachring, and have axes passing through a central point of the bottom of thecavity.